The invention relates to an avalanche photodiode for detecting radiation.
Sadygov Z.: “Three advanced designs of micro-pixel avalanche photodiodes: Their present status, maximum possibilities and limitations”, Nuclear Instruments and Methods in Physics Research A 567 (2006) 70-73 discloses this type of avalanche photodiode which can be used for the purpose of detecting radiation. Located in this case in a semiconductor substrate is an avalanche region which is formed by means of a pn-transition between a cathode layer and an anode layer and in which the radiation to be detected triggers an avalanche breakdown. Furthermore, a quenching resistor is provided in this case which is connected in series to the avalanche region and has the task of terminating a radiation-generated avalanche breakdown, in that the voltage drop across the quenching resistor lowers the current until the charge carrier multiplication dies off in the avalanche region.
In one variation of this known avalanche photodiode, the quench resistor is situated partially on the radiation entry window and must still be at least partially contacted with thin metallic layers. In this case, the quenching resistor thus forms an obstacle for the radiation which is to be detected, whereby the detection efficiency deteriorates drastically particularly for ultraviolet (UV) and blue light.
In another variation, it is provided in the above-mentioned publication by Sadygov et al. that the quenching resistor is integrated together with a coupling capacitor into the semiconductor substrate (bulk), wherein the avalanche region is located deeply buried in the semiconductor substrate on an epitaxial layer boundary surface.
On the one hand, this is associated, as stated in the said publication, with technological difficulties, as deep ion implantation and epitaxial growth are required on highly pure silicon wafers.
On the other hand, a common quenching resistor is provided in each case for many avalanche photodiodes, so that upon activation of a diode large neighboring regions become insensitive.
A further problem relating to the known avalanche photodiodes is based upon the fact that radiation detectors are operated in general in an environment exposed to radiation. Therefore, comprehensive preliminary tests are required especially in the case of space-related applications, in order to ensure adequate long-term stability of the avalanche photodiodes. Although silicon as a semiconductor material for avalanche photodiodes has the significant advantage of a passivating oxide which has excellent dielectric properties and can be produced with relatively small defects and warping on the silicon-silicon dioxide boundary surface, this boundary surface still constitutes the most sensitive part with respect to ionizing radiation. Both the additionally generated boundary surface charges and the boundary surface generation current (leakage current) can exceed the initial values by orders of magnitude prior to irradiation. Primarily, the isolation structures of the conventional radiation detectors frequently fail for this reason. Therefore, detectors with a higher level of beam resistance are desirable.
Furthermore, with respect to the prior art reference is to be made to EP 1 840 967 A1 and JP 09-64398 AA, EP 1 755 171 A1, US 2006/0249747 A1 and U.S. Pat. No. 6,222,209 B1.
Therefore, the object of the invention is to improve the above-described conventional avalanche photodiode accordingly.
Preferably, the avalanche photodiode in accordance with the invention should be able to be arranged in matrix form in a radiation detector, in order to detect individual optical photons.
Furthermore, the avalanche photodiode in accordance with the invention should be producible in the simplest manner possible.
It is also desirable that the avalanche photodiode in accordance with the invention is as resistant as possible to ionizing beams.
Furthermore, the avalanche photodiode in accordance with the invention should have a high quantum efficiency and a high degree of sensitivity in the ultraviolet and blue spectral range.
The aforementioned objects are achieved by means of an avalanche photodiode in accordance with the invention.